An excavator with an electric work element that is driven by an electric motor or an electric actuator generally includes an electrical energy storage unit for supplying electric power to the electric work element. A hybrid excavator with a motor generator for assisting an engine (assist motor) generally includes an electrical energy storage unit including an electrical energy storage device or a battery for storing the electric power obtained by driving the motor generator. The assist motor is driven by the electric power from the electric energy storage unit to assist the engine. Also, the assist motor is driven by the power of the engine to generate power. The generated electric power is stored in the electrical energy storage device or battery of the electrical energy storage unit.
To supply electric power as requested, the electrical energy storage device or battery of the electrical energy storage unit is controlled so that its charge rate (e.g., SOC) may always be at least a certain value. Thus, when the charge rate (SOC) is dramatically decreased, a large charge current may be supplied to the electrical storage unit in an attempt to rapidly increase the charge rate (SOC). Because the electrical energy storage device or battery has internal resistance, heat generation may occur when a charge current is supplied thereby resulting in internal resistance power loss. Because the internal resistance power loss is proportional to the square of the current, the internal resistance power loss abruptly increases when the charge current is increased. Thus, to reduce the internal resistance and make efficient use of the electrical energy storage device or battery, the charge current is preferably arranged to be as small as possible.
In this respect, for example, Japanese Unexamined Patent Publication No. 2009-114653 discloses supplying electric power to a battery of an electrically-driven construction machinery from an external alternator via a power leveling unit. The power leveling unit limits the peak power of the electric power from the alternator to a predetermined upper limit value so as to level the electric power and supplies the leveled electric power to the battery of the construction machinery. That is, an upper limit value for the charge current is set and the charge current is prevented from exceeding this upper limit value.